1. Technical Field
The present invention relates to a motor, and in particular to a winding structure of a winding in a motor.
2. Background Art
A motor comprises a stator and a rotor. Of these, a winding is wound around the stator, and an electric current is applied to the winding to generate a torque and rotate the rotor. The winding wound around the stator is placed in slots within the stator. The number of windings passing inside the slots is normally set such that a certain number of windings are provided in parallel, and are wound through the same slot for a predetermined number of times. The number of lines to be provided in parallel is called a number of parallels, and the number of times the windings are wound in the same slot for a plurality of times is called a number of turns.
The number of turns will now be described with reference to FIGS. 4 and 5. FIG. 4 is a schematic diagram showing a motor. In FIG. 4, a portion shown with M represents the motor, and three lines which extend from this portion represent three-phase lead lines extending from the motor. A line within the motor M forming a spiral shape represents a winding wound around the motor. Of the three-phase windings, a winding method of the winding between U and X which is a winding of one phase will be described with reference to FIG. 5. FIG. 5 is a schematic diagram showing an example winding method of windings when the number of slots=12 and the number of turns=3. In FIG. 5, the portions represented by S1 and S2 are slots within the motor. C1 and C2 represent coils, of the windings which are wound, which form blocks. As the motor shown in FIG. 5 has 12 slots, the number of slots for one phase is ⅓ of 12, or 4 slots. In addition, because the windings are wound between two slots, the number of coils is 2. In each coil, the same winding is wound in each slot passing for 3 times, and the winding moves to the next slot and the next coil is wound. These processes are repeated so that 3 turns of windings are wound. Similarly, the portion between V and Y and the portion between W and Z are wound, so that the windings of the motor M are wound.
The number of turns is a constant determined by a specification of the motor such as the rotational speed. In general, a voltage generated in a winding in a rotor is known to be proportional to the number of turns of the winding and an amount of change, with respect to time, of a magnetic flux crossing the winding. Because of this, when the current supplied to the motor is constant, the generated torque is increased as the number of turns is increased. Meanwhile, however, when the number of turns is increased, the voltage generated in the winding is also increased. In addition, as the rotational speed is increased, the amount of change of the magnetic flux crossing the winding is also increased in proportion to the rotational speed, and, thus, when the number of turns is increased, the voltage generated in the winding during high rotational speed is increased. If the voltage generated in the winding reaches a power supply voltage of an amplifier connected to the motor, it becomes impossible to supply current from the amplifier to the motor, and the motor cannot be operated. Therefore, the number of turns of the windings is determined such that the motor satisfies the target specification and, at the same time, a desired output can be obtained by applying current within a range where the voltage generated in the winding does not become larger than a predetermined allowable value.
The number of parallels is determined based on the number of turns determined as described above. A total number of windings included in the slot of the stator is the number of turns times the number of parallels. A ratio of the total cross sectional area of the winding over the cross sectional area of the slot is called a lamination factor. Because the slot shape is determined in advance, the lamination factor is limited within a certain value when the windings are inserted into the motor. In the motor, in order to reduce generation of heat, it is desired to minimize the resistance value of the winding, and, consequently, to maximize the thickness of the winding. However, because the lamination factor is limited within a certain value as described above, there is a problem on how the windings are inserted within the determined lamination factor. In this process, the line size of the winding and the number of parallels are adjusted so that the lamination factor is as close to the limitation value as possible. In this manner, the number of turns and the number of parallels of the winding in the motor are determined.
As described, when the winding is designed, the number of turns is determined based on the output which can be achieved by the motor and the magnitude of the voltage generated in the winding. In this configuration, for example, a case may occur where, with 2 turns, the voltage generated in the winding is within the allowable value but the torque that can be generated is small and the specification cannot be satisfied because the number of turns is small, and, with 3 turns, although the specification can be satisfied, the voltage generated in the winding around the maximum rotational speed exceeds the allowable value. In other words, in this example case, if a specification having an output line diagram as shown in FIG. 6, with a base rotational speed of nb, a maximum rotational speed of nt, and an output of p0, is to be satisfied, with the number of turns of the winding being 2 turns, although the voltage generated in the winding can be set within the allowable value for all rotational speed regions when the output of p0 is generated, the generated torque is small because of the small number of turns, the output of p0 cannot be generated at the rotational speed of nb, and the specification cannot be satisfied with 2 turns. On the other hand, if the number of turns is set to 3 turns, the generated torque is approximately 1.5 times that of the configuration with 2 turns, because the number of turns is 1.5 times that of the configuration with 2 turns, and the output of p0 can be generated at the rotational speed nb, but the voltage generated in the winding exceeds the allowable value when the output of p0 is to be generated at a rotational speed greater than a rotational speed nc. Therefore, as shown in FIG. 7, there is employed a configuration where the output is reduced from p0 at the rotational speed from nc to nt, in order to reduce the current to be applied, and to limit the voltage generated in the winding to a value within the allowable value.
In this manner, in the case where, for example, a configuration of 2 turns is not sufficient and a configuration of 3 turns results in an extra margin in order to generate the output of p0 at nb, and the configuration of 3 turns results in the voltage generated in the winding exceeding the allowable value with the rotational speed exceeding nc and the configuration of 2 turns results in an extra margin of the voltage generated in the winding with respect to the allowable value even if the output of p0 is generated to the maximum rotational speed nt, it can be expected, through calculation, that, with an intermediate number of rotations; for example, 2.5 turns at the midpoint between 2 and 3 turns, the voltage generated in the winding would be within the allowable value and the output would satisfy the specification. However, only an integer can be selected for the number of turns, and, thus, one of 2 turns and 3 turns must be selected. As a result, in many cases, a configuration of 3 turns is selected and the amount of application of current at a high rotational speed is reduced to intentionally reduce the output at the high rotational speed.
However, in this case, although the motor has a capability to satisfy the specification, the capability of the motor cannot be fully utilized due to limitation that the number of turns must be an integer.